The use of fossil fuel in gas turbine engines results in the combustion products consisting of carbon dioxide, water vapor, oxides of nitrogen, carbon monoxide, unburned hydrocarbons, oxides of sulfur and particulates. Of these above products, carbon dioxide and water vapor are generally not considered objectionable. In most applications, governmental imposed regulations are further restricting the remainder of the species, mentioned above, emitted in the exhaust gases.
The majority of the products of combustion emitted in the exhaust can be controlled by design modifications, cleanup of exhaust gases and/or regulating the quality of fuel used. For example, particulates in the engine exhaust have been controlled either by design modifications to the combustor and fuel injectors or by removing them by traps and filters. Sulfur oxides are normally controlled by the selection of fuels that are low in total sulfur. This leaves nitrogen oxides, carbon monoxide and unburned hydrocarbons as the emissions of primary concern in the exhaust gases emitted from the gas turbine engine.
The principal mechanism for the formation of oxides of nitrogen involves the direct oxidation of atmospheric nitrogen. The rate of formation of oxides of nitrogen by this mechanism depends mostly upon the flame temperature and to some degree upon the concentration of the reactants and, consequently, a small reduction in flame temperature can result in a large reduction in the nitrogen oxides.
Attempts to control NOx emissions by regulating the local flame temperature have adopted the use of water or steam injection. This system increases cost due to the additional equipment, such as pumps, lines and storage reservoir. Furthermore, in areas where a supply of water is not readily available the cost and labor to bring in water basically makes this option undesirable.
In an attempt to reduce NOx emissions without incurring increase in operational cost caused by water or steam injection, gas turbine combustion systems have utilized a premix approach. The premix system and nozzles used therewith are examples of attempts to reduce the emissions of oxides of nitrogen. The systems and nozzles described above although generally efficiently controlling the emissions of oxides of nitrogen emitted from the engine exhaust have failed to compensate for combustion induced pressure oscillation problems resulting from the premix approach.